Due to the inherent safety of IT power supply systems (French: Isolé Terre—IT), a continuous power supply of the user supplied by the IT power supply system can be ensured, even if a first insulation fault has occurred, since the active parts of the IT power supply system are separated from the ground potential—with respect to “ground”—and a closed electric circuit cannot be formed in this first fault case. A first insulation fault occurs, for example, if there is an unintended electric connection between an active conductor of the IT power supply system and the conductive casing of an operating part (user).
Provided that the insulation state of the IT power supply system is continuously monitored by an insulation monitoring device, the IT power supply system can still be operated without a time limitation, even when a first fault has occurred, however, the first fault is to be removed as quickly as practically possible according to the recommendation of standards DIN VDE 0100-410 and IEC 60364-4-41, respectively.
A first insulation fault is recognized and reported by the insulation monitoring device. This report starts an insulation fault location by a test current being produced by a test current generator of an insulation fault location system and being supplied into the IT power supply system at a central location. In the branches of the IT power supply system involved in the fault location, a test current sensor detects whether a significant test current portion flows into the respective branch.
The signals of the test current sensor are centrally detected in an analyzing device and an insulation fault localization is carried out based on these signals.
According to the state of the art, the test current parameters, in particular the test current amplitude as well as the test current pulse duration with a pulse-shaped test current, of the test current produced and supplied by the insulation fault location system are set once when planning and installing the safety-critical measures for the IT power supply system. Thereby, a compromise is to be made between the following requirements: (1) preventing risk to persons and preventing fire hazards by the test current, (2) no impediment of the function of the IT power supply system and the connected operating parts by the test current and (3) localization of even high-resistance insulation faults.
In practice, the test current is limited to a few 10 mA or even only a few mA in order to safely fulfill the requirements (1) and (2).
In particular in widely branched IT power supply systems, problems regarding the size and/or the distribution of network leakage capacitances can occur. These network leakage capacitances are mostly electrically parallel to the fault resistances to be detected. A portion of the test current therefore flows through these capacitances and, when the test current is set too low, leads to high-resistance fault resistances not being detected, in a worst case scenario. The distribution of the test current on multiple faulty branches in a badly maintained IT power supply system poses another problem. Multiple faults are difficult to detect with small test current amplitudes, since the low test current distributes itself on multiple fault resistances and the sensitivity threshold of the test current sensors can fall short. Ultimately, disturbing elements can occur in IT power supply systems which impede or prevent an insulation fault location, should the test current portion in the faulty branch is so low that it is hidden by disturbing signals and cannot be detected or detected sufficiently exact by the test current sensor.
The circumstances mentioned above can therefore result in an insulation fault not being located.
Known measures for solving these problems consist of a manual adjustment of the test current parameter, on the one hand, for example by manually increasing the test current amplitude, as far as this is possible in the installed insulation fault location system. However, since the estimation of a maximal admissible test current amplitude requires considerable expertise in a present fault case, service personnel trained specifically in this area is employed for this purpose. Thus, the maintenance measures are delayed and increase in price in an unfavorable way.
As an additional measure, on the other hand, the insulation fault location system or components of the installed insulation fault location system can be substituted by a system having a higher test current amplitude, provided such a system is available on the market. A substitution of such components in critical IT power supply systems often requires a renewed inspection by an expert.
Furthermore, the fact that the test current amplitude of currently available insulation fault location systems is limited to low values of only a few 10 mA by the producer is still another disadvantage. Therefore, the only possibility that remains in the problematic cases mentioned above is to carry out the fault location without the help of insulation fault location systems, for example by shutting down branches (sub-systems).
The solutions mentioned above are therefore not always satisfactory in practice. Moreover, a delay in localizing and removing a first fault can lead to an unnecessary strain on the IT power supply system or even to a shutting down of the power supply.
Therefore, the object of the present invention is to enhance a method and a device for insulation fault location in such a way that a reliable insulation fault location can be carried out in an economically feasible way, wherein at the same time a high electric safety is ensured.